Physical vapor deposition system with a source of isotropic ion velocity distribution at the wafer surface

ABSTRACT

In a plasma enhanced physical vapor deposition of a material onto workpiece, a metal target faces the workpiece across a target-to-workpiece gap less than a diameter of the workpiece. A carrier gas is introduced into the chamber and gas pressure in the chamber is maintained above a threshold pressure at which mean free path is less than 5% of the gap. RF plasma source power from a VHF generator is applied to the target to generate a capacitively coupled plasma at the target, the VHF generator having a frequency exceeding 30 MHz. The plasma is extended across the gap to the workpiece by providing through the workpiece a first VHF ground return path at the frequency of the VHF generator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 12/077,067, filed on Mar. 14, 2008, the entiredisclosure of which is incorporated by reference.

BACKGROUND

Plasma enhanced physical vapor deposition (PECVD) processes are used todeposit metal films such as copper onto semiconductor wafers to formelectrical interconnections. A high level of D.C. power is applied to acopper target overlying the wafer in the presence of a carrier gas, suchas argon. Plasma source power may be applied via a coil antennasurrounding the chamber. PECVD processes typically rely upon a verynarrow angular distribution of ion velocity to deposit metal ontosidewalls and floors of high aspect ratio openings. One problem is howto deposit sufficient material on the sidewalls relative to the amountdeposited on the floors. Another problem is avoiding pinch-off of theopening due to faster deposition near the top edge of the opening. Asminiaturization of feature sizes has progressed, the aspect ratio(depth/width) of a typical opening has increased, with microelectronicfeature sizes having now been reduced to about 22 nanometers. Withgreater miniaturization, it has become more difficult to achieve minimumdeposition thickness on the sidewall for a given deposition thickness onthe floor or bottom of each opening. The increased aspect ratio of thetypical opening has been addressed by further narrowing of the ionvelocity angular distribution, through ever-increasing wafer-to-sputtertarget distance and ever lower chamber pressures, e.g., less than 1 mT(to avoid velocity profile widening by collisions). This has given riseto a problem observed in thin film features near the edge of the wafer:At extremely small feature sizes, a portion of each high aspect ratioopening sidewall is shadowed from a major portion of the target becauseof the greater wafer-to-target gap required to meet the decreasingfeature size. This shadowing effect, most pronounced near the waferedge, makes it difficult if not impossible to reach a minimum depositionthickness on the shadowed portion of the side wall. With furtherminiaturization, it has seemed a further decrease and chamber pressure(e.g., below 1 mT) and a further increase in wafer-sputter target gapwould be required, which would exacerbate the foregoing problems.

One technique employed to supplement the side wall deposition thicknessis to deposit an excess amount of the metal (e.g., Cu) on the floor ofeach opening and then re-sputter a portion of this excess on the openingside wall. This technique has not completely solved the shadowingproblem and moreover represents an extra step in the process and alimitation on productivity.

A related problem is that the sputter target (e.g., copper) must bedriven at a high level of D.C. power (e.g., in the range of kW) toensure an adequate flow of ions to the wafer. Such a high level of D.C.power rapidly consumes the target (driving up costs) and produces anextremely high deposition rate so that the entire process is completedin less than five seconds. This time is about 40% of the time requiredfor the RF source power impedance match to equilibrate following plasmaignition, so that about 40% of the process is performed prior tostabilization of the impedance match and delivered power.

SUMMARY

A method is provided for performing physical vapor deposition on aworkpiece in a reactor chamber. The method includes providing a targetcomprising a metallic element and having a surface facing the workpiece,and establishing a target-to-workpiece gap less than a diameter of theworkpiece. A carrier gas is introduced into the chamber and gas pressurein the chamber is maintained above a threshold pressure at which meanfree path is less than 5% of the gap. RF plasma source power from a VHFgenerator is applied to the target to generate a capacitively coupledplasma at the target, the VHF generator having a frequency exceeding 30MHz. The method further includes extending the plasma across the gap tothe workpiece by providing through the workpiece a first VHF groundreturn path at the frequency of the VHF generator.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the exemplary embodiments of the presentinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings. It is to be appreciated that certain well knownprocesses are not discussed herein in order to not obscure theinvention.

FIG. 1 is a diagram of a plasma reactor in accordance with a firstaspect.

FIG. 2 is a graph of a random or near-isotropic velocity distribution ofions at the wafer surface attained in the reactor of FIG. 1.

FIG. 3 is a graph depicting angular velocities in the distribution ofFIG. 2.

FIG. 4 is block diagram depicting method in accordance with oneembodiment.

FIG. 5 is a diagram of a plasma reactor in accordance with a secondaspect.

FIGS. 6A and 6B depict alternative embodiments of variable ground returnimpedance elements in the reactor of FIG. 1 or FIG. 5.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation. It is to be noted, however, that the appendeddrawings illustrate only exemplary embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

DETAILED DESCRIPTION

In one embodiment, a PEPVD process provides complete uniform sidewallcoverage at feature sizes of 25 nm and below (e.g., 18 nm) free of thenon-uniformities caused by shadowing. The PEPVD process of thisembodiment is carried out with VHF plasma source power on the sputtertarget to create a capacitively coupled RF plasma at the target. Theprocess further employs a very low (or no) D.C. power on the sputtertarget. With the low D.C. power level on the target, the process may beperformed over a period of time that is longer than the settling time ofthe impedance match elements, and sufficiently long for good processcontrol and chamber matching. These advantages are realized by usinghigh chamber pressure to at least reduce or totally eradicate theneutral directionality from the metal target to the wafer. If, in factthe remaining deposition were to be accomplished from neutrals emanatingfrom the vacuum gap of the mean free path above the wafer, the featureswould totally be pinched off. However, the RF power maintains asignificant density of metal ions in the plasma, making them availablefor attraction to the wafer by electric field. The electric field can bederived by either a residual RF field from the VHF plasma source at theceiling (target) or by a small RF bias power applied to the waferdirectly. This then creates a predominantly vertical ion velocitydistribution with some perpendicular (horizontal) velocity component forside wall coverage. The result is that the side wall and floor of eachhigh aspect ratio opening is conformally covered with the sputteredmaterial from the target. In summary, the source provides a nearlyisotropic distribution of neutral velocity and a distribution of ionvelocity including a large vertical component and a relatively smallernon-vertical or horizontal component. The isotropic neutral velocitydistribution and somewhat broadened but predominantly vertical ionvelocity distribution at the wafer surface is realized by maintainingthe chamber at an extremely high pressure (e.g., 100 mT) to ensure anion collision mean free path that is 1/20^(th) of the wafer-to-sputtertarget gap. A high flux of sputtered ions at the wafer is realized by:(1) minimizing the wafer-to-sputter target gap to a fraction of thewafer diameter, (2) generating a VHF capacitively coupled plasma at thesputter target (as mentioned above) and (3) extending the capacitivelycoupled plasma down to the wafer. The plasma is extended down to thewafer by providing an attractive VHF ground return path through thewafer. The process window is expanded by reducing (or possiblyeliminating) the D.C. power applied to the sputter target, so that thetarget consumption rate is reduced and the process is less abrupt. Thisreduction in D.C. target power without loss of requisite sputtering ismade possible by the high density plasma generated at the target byapplication of VHF power to the target and by the reducedwafer-to-sputter target gap.

Undesirable ion bombardment by ions of the carrier gas (e.g., argonions) is suppressed by selectively favoring the desired sputter targetions (e.g., copper ions) at the plasma sheath overlying the wafersurface. This selection is made by maintaining the wafer bias voltagebelow an upper threshold voltage (e.g., 300 volts) above which carriergas (e.g., argon) ions interact with or damage thin film structures onthe wafer. However, the wafer bias voltage is maintained above a lowerthreshold voltage (e.g., 10-50 volts) above which the sputter targetmaterial (e.g., copper) ions deposit on the wafer surface. Such a lowwafer bias voltage is achieved by differential control of VHF groundreturn path impedances for the VHF source power through: (a) the waferand (b) the chamber side wall, respectively. Decreasing the VHF groundreturn path impedance through the sidewall tends to decrease the waferbias voltage. This control is provided by independent variable impedanceelements governing ground return path impedances through the side walland through the wafer, respectively.

The wafer bias voltage is also minimized as follows: The VHF sourcepower applied to the target creates a modest positive bias voltage onthe wafer, in the absence of any other applied RF power. This positivewafer bias may be offset by applying a small amount of optional lowfrequency (LF) RF bias power to the wafer. The LF bias power tends tocontribute a negative bias voltage on the wafer, so that its negativecontribution may be adjusted or balanced against the positivecontribution of the VHF source power to produce a net wafer bias voltagethat is close to zero, if desired, or as small as desired. Specifically,the wafer bias voltage is reduced well below the carrier gas ionbombardment bias threshold referred to above.

The foregoing process has been discussed above with reference to a PEPVDprocess for depositing copper. However, the process can be used todeposit a wide range of materials other than copper. For example, thesputter target may be titanium, tungsten, tin (or other suitablemetallic materials or alloys) for PEPVD deposition of the metallictarget material (e.g., titanium, tungsten or tin). Moreover, a metallic(e.g., titanium) target may be employed with a nitrogen process gas fordeposition of titanium nitride or other metallic nitride, where acarrier gas (argon) is used for plasma ignition and is then replaced bynitrogen for the metallic (titanium) nitride deposition.

Referring to FIG. 1, a PEPVD reactor in accordance with a firstembodiment includes a vacuum chamber 100 defined by a cylindricalsidewall 102, a ceiling 104 and floor 106, the chamber containing awafer support 108 for holding a wafer 110 in facing relationship withthe ceiling 104. A metal sputtering target 112 is supported on theinterior surface of the ceiling 104. A vacuum pump 114 maintainspressure within the chamber 100 at a desired sub-atmospheric value. Aconventional rotating magnet assembly or “magnetron” 116 of the typewell-known in the art overlies the ceiling 104 directly above thesputtering target 112. A process gas supply 118 furnishes a carrier gassuch as argon into the chamber 100 through a gas injection apparatus120, which may be a gas injection nozzle, an array of nozzles or a gasdistribution ring.

The wafer support 108 may embody an electrostatic chuck including agrounded conductive base 108-1, an overlying dielectric puck 108-3having a wafer support surface 108-5, and an electrode or conductivemesh 108-7 inside the puck 108-3 and separated from the wafer supportsurface by a thin layer 108-9 of the dielectric puck 108-3.

A D.C. power supply 122 is connected to the center of the target 112through a central aperture 116-1 in the magnetron 116. An RF plasmasource power generator 124 having a VHF frequency is coupled through aVHF impedance match 126 to the center of the target 112 through thecenter magnetron aperture 116-1. A D.C. chucking voltage supply 128 isconnected to the chuck electrode 108-7.

In operation, the VHF power generator 124 provides about 4 kW of plasmasource power to support a capacitively coupled plasma at the target 112initially consisting of ions of the carrier gas. This plasma sputtersthe target 112 to generate free target (e.g., copper) atoms which becomeionized in the plasma, the rotation of the magnetic fields of themagnetron 116 helping to distribute the consumption of the target 112and promote ionization near the target 112. The reactor includesfeatures that enable the plasma generated at the target 112 to reach thewafer 110. In accordance with one such feature, the plasma formed at thetarget 112 by the capacitively coupled VHF power from the generator 124is made to extend down to the wafer 110 by providing an attractive VHFground return path through the wafer 110 (i.e., through the wafersupport 108). For this purpose, a variable impedance element 130 iscoupled between the electrode 108-7 and ground. Other than thisconnection, the electrode 108-7 is insulated from ground, so that theimpedance element 130 provides the only connection between the electrode108-7 and ground. The variable impedance element 130 in oneimplementation consists of a series reactance 132 such as a capacitor,and variable parallel reactances 134, 136, of which the reactance 134may be a variable capacitor and the reactance 136 may be a variableinductor. The reactances 132, 134, 136 are selected to provide animpedance at the frequency of the VHF generator 124 that allows currentat that frequency to flow from the electrode 108-7 to ground.

Another feature that enables the plasma to extend to from the target tothe wafer is a reduction in the gap between the wafer 110 and the target112. The wafer-target gap is reduced to a distance less than or afraction of the wafer diameter. For example, the gap may be ⅕^(th) ofthe wafer diameter. For a 300 mm wafer diameter, the wafer-target gapmay be 60 mm.

The pump 114 is set to provide a high chamber pressure (e.g., 50-200mT). The chamber pressure is sufficiently high to set the mean free pathto less than 1/20^(th) of the length of the wafer-to-target gap. Thespace between the target 112 and the wafer 110 is empty, i.e., free ofother apparatus, to ensure maximum dispersion of angular velocitydistribution of the neutrals by the large number of collisions in theirtransit from the target 112 to the wafer 110. The resulting angulardistribution of velocity of the neutrals in the plasma is very broad(nearly uniform) at the wafer surface within a hemispherical angularrange from 0° (normal to the wafer surface) up to nearly 90° (parallelto the wafer surface). The ions have a less uniform angular distributionthat peaks at the perpendicular direction, due to the attractionpresented by the wafer bias voltage. FIG. 2 is a simplified diagramdepicting the distribution of ion population as a function of directionin a cone of ion trajectories depicted in FIG. 3, showing a peak at theperpendicular direction. This peak has been broadened from a highvariance (e.g., 0.8) to a variance (e.g., 0.2) by the large number ofcollisions, mainly with neutrals, that the ions experience within thewafer-target gap. These collisions compete with the electric field toreduce the sharp peak of ion velocity distribution about the verticaland provide a small component of non-vertical (e.g., horizontal) ionvelocity. This broadening of the ion angular trajectory distribution iscombined with the nearly isotropic angular distribution of neutralvelocities. This combination improves the uniformity or conformality ofthe deposited film. As a result of the broadened angular distribution ofion velocity at the wafer surface, coupled with the relatively smallwafer-to-target gap, metal is deposited on interior surfaces of highaspect ratio openings in the surface of the wafer with very highconformality and uniform thickness.

The capacitively coupled plasma produced at the target 112 contains ionsfrom the carrier gas (e.g., argon ions) and ions from the target (e.g.,copper ions). The copper ions require a relatively low plasma biasvoltage on the wafer to deposit on the wafer surface, typically around50 volts or less. The argon ions are more volatile than the copper ions,and at the low sheath voltage tend to elastically collide with thefeatures on the wafer surface and disperse, rather than impartingdamage. At slightly greater bias voltage levels (e.g., 300 volts), theargon ions collide inelastically with thin film features on the waferand damage them. Therefore, ideal results can be achieved by limitingthe wafer bias voltage to about 50 volts or less, for example. Theproblem is how to limit the wafer bias voltage to such a low level.

A first feature for limiting wafer bias voltage is one that diverts aselected portion of the plasma ions away from the wafer 110 to thechamber sidewall 102 (which is formed of a metal). This feature employsthe variable VHF ground return impedance element 130 coupled to thechuck electrode 180-7 and, in addition, a second variable VHF groundreturn impedance element 140 coupled to the side wall 102. The sidewallvariable VHF impedance element 140 has a structure similar to that ofthe element 130, and (in one implementation) includes a series capacitor142, a variable parallel capacitor 144 and a variable inductor 146. Thesidewall variable impedance element 140 is connected between thesidewall 102 and ground, the sidewall 102 being insulated from groundwith the exception of this connection. The impedances of the twoelements 130, 140 are independently adjustable, and determine theapportionment of the plasma current between the wafer 110 and thesidewall 102. Adjustment of the impedances of the elements 130, 140 isperformed to reduce the wafer bias voltage down to a low level and toaccurately select that level. For example, a reduction in wafer biasvoltage may be obtained by increasing the resistance at the VHF sourcepower generator frequency presented by the chuck electrode impedanceelement 130 (rendering the VHF ground return path through the wafer 110less attractive) while reducing the resistance at the VHF source powergenerator frequency presented by the chamber sidewall impedance element140 (rendering the VHF ground return path through the sidewall 102 moreattractive). The relative impedances of the two impedance elements 130,140 at the VHF source power frequency for a given apportionment ofplasma current depends upon the relative areas of the wafer 110 and theconductive side wall 102.

In order to reduce the conductance through a selected one of twovariable impedance elements 130, 140, the impedance may be chosen toeither behave as a very high resistance or open circuit at the frequencyof the VHF generator 124, or to behave as a very low resistance or shortcircuit at harmonics of the frequency of the VHF generators (e.g.,2^(nd), 3^(rd), 4^(th) harmonics). Such behavior at the harmonicfrequencies may be implemented in either or both of the impedanceelements 130, 140 with the addition of further adjustable tank circuitsfor each of the harmonics of interest. This may be accomplished byadding more variable reactances to the impedance elements 130, 140 toachieve the desired filter or pass band characteristics. For example,referring to FIG. 6A, the variable impedance element 130 may furtherinclude variable resonant circuits 130-1, 130-2, 130-3 that may be tunedto provide selected impedances at the second, third and fourthharmonics, respectively. Referring to FIG. 6B, the variable impedanceelement 140 may further include variable resonant circuits 140-1, 140-2,140-3 that may be tuned to provide selected impedances at the second,third and fourth harmonics, respectively.

A second feature for limiting wafer bias voltage exploits the tendencyof the VHF source power applied to the target 112 to produce a modestpositive bias voltage on the wafer 110 (in the absence of any other RFpower being applied). This second feature involves coupling an optionallow frequency RF bias power generator 150 through an impedance match 155to the chuck electrode 108-7. As RF bias power from the generator 150 isincreased from zero, the wafer bias voltage, which is initially positiveunder the influence of the VHF source power from the VHF generator 124,is shifted down and at some point crosses zero and becomes negative. Bycarefully adjusting the output power level of the bias power generator150 to a small power level (e.g., about 0.1 kW), the wafer bias voltagecan be set to a very small value (e.g., to less than 50 volts).

Since the VHF source power generator 124 provides plasma source powerfor the generation of plasma ions near the target 112, the D.C. powersource 122 is not the sole source of power for plasma generation. Thedemands on the D.C. power source 122 are further lessened because thereduced wafer-target gap reduces loss of plasma density between thetarget 112 and the wafer 110. Therefore, the D.C. power level of theD.C. supply 122 may be reduced from the conventional level (e.g., 35-40kW) to as low as 2 kW. This feature reduces the sputtering rate of thetarget 112 and therefore reduces the consumption of the target, cost ofoperation and thermal load on the entire system. Moreover, it reducesthe deposition rate on the wafer 110. At the higher D.C. power level(e.g., 38 kW) the deposition rate was extremely high, and the depositiontime had to be limited to about 5 seconds for a typical copper filmdeposition thickness, of which the first 2 seconds were spent by theimpedance match 126 reaching equilibrium or stability following plasmaignition. At the new (reduced) D.C. power level (of a few kW or less),the deposition time may be on the order of 30 seconds, so that theimpedance match 126 is stable for a very high percentage of the processtime.

The increase in uniformity of the metal coating on interior surfaces ofhigh aspect ratio openings increases the process window over which thereactor may be operated. In the prior art, the metal deposition on theinterior surfaces of high aspect ratio openings was highly non-uniform,which allowed for only a very small wafer-to-wafer variation inperformance and a very narrow process window within which adequate metalcoating could be realized for all internal surfaces of a high aspectratio opening. Furthermore, the inadequate deposition thickness onsidewalls of high aspect ratio openings required the performance of asecond step following deposition, namely a re-sputtering step in whichexcess material deposited on the floor or bottom of an opening istransferred to the sidewall. The re-sputtering step has typicallyrequired an excess thickness to be deposited on the floor of the highaspect ratio opening. With the present embodiment, the improveduniformity of the deposition on the floor and sidewall of the openingeliminates the need for re-sputtering and the need for excess thicknesson the bottom of the opening. This increases productivity and reducesthe amount of material that must be removed when opening a via throughthe floor of the high aspect ratio opening.

The total power applied to the chamber is reduced in the embodiment ofFIG. 1 from a convention PEPVD process, as can be seen in the followingtable (Table I).

TABLE I conventional reactor reactor of FIG. 1 D. C. power 38 kW 2 kW RFsource power 1 kW 4 kW RF bias power 1 kW 0.1 kW TOTAL POWER 40 kW 6.1kW gap 390 mm 60 mm pressure 0.5 mT 100 mT mean free path 550 mm 2.7 mmcollisions/transit 0.30 22.22

The disclosed process overcomes the problem of shadowing during PECVDprocesses for metal deposition in high aspect ratio openings. Suchopenings may be on the order of 22 nanometers in diameter and have a 7:1aspect ratio (height:width). Prior to the invention, PECVD metaldeposition in such openings near the edge of a 300 mm wafer exhibitedgreat non-uniformity due to shadowing effects. The ratio between themetal deposited on radially inner and outer sides of the openingsidewall was as great as 50:1, corresponding to a highly non-uniformsidewall deposition. With the process disclosed above, it is improved tonearly 1:1, a uniform sidewall deposition.

FIG. 4 depicts a method in accordance with one aspect. A target isprovided having a material (such as copper) requiring a low wafer biasvoltage (e.g., 10-50 volts) for deposition (block 410 of FIG. 4). Acarrier gas such as argon is introduced into the chamber that tends notto produce ion bombardment damage at a low wafer bias voltage (block412). A plasma is generated at the target by capacitively coupling VHFsource power using the target as an electrode (block 414). This plasmais extended down to the wafer by providing a VHF ground return paththrough the wafer (block 416), and by establishing a narrowwafer-to-target gap (block 418). The chamber pressure is maintained at asufficiently high level (e.g., 100 mT) to ensure an ion mean free pathlength that is less than 1/20^(th) of the wafer-target gap (block 420),to establish a random or nearly isotropic ion velocity distribution atthe wafer surface (block 422). The wafer bias voltage is minimized byapportioning the plasma between a ground return path through the waferat a first impedance and a ground return path through the chambersidewall at a second impedance (block 424). The wafer bias voltage isfurther minimized by offsetting a positive bias voltage induced by theVHF source power with a negative bias voltage induced by an LF biaspower generator (block 426). The steps of blocks 424 and 426 areperformed so as to keep the wafer bias below a threshold at which thecarrier gas ions can inflict ion bombardment damage but above thenecessary threshold to deposit the target material.

FIG. 5 depicts a modification of the embodiment of FIG. 1, in which thesputter target D.C. power supply 122 is eliminated and in which theoptional RF bias power generator 150 and match 155 are not present. Inthe embodiment of FIG. 5, the wafer-target gap is sufficiently small andthe VHF source power generator 124 has a sufficiently high output powerlevel to provide, with the attraction of a VHF return path through thewafer 110, the plasma ion density at the target 112 necessary for adesired sputtering rate at the target and deposition rate at the wafer.In the absence of the D.C. power on the target, the target consumptionis slower and the thermal load on the system is less and the processwindow is wider.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. An apparatus for performing physical vapordeposition, comprising: a chamber; a target comprising a metallicelement disposed in the chamber, wherein the target provides a firstelectrode for the chamber; a support having a top surface to support aworkpiece having a diameter in the chamber, wherein the support includesa second electrode, and wherein the support is positioned to hold theworkpiece such that a target-to-workpiece gap is less than one-fifth ofa diameter of the workpiece; a gas supply configured to introduce acarrier gas into the chamber and maintain a gas pressure in the chamberabove a threshold pressure at which a mean free path is less than 5% ofthe gap; a VHF generator having a frequency exceeding 30 MHz to applypower to the target to generate a capacitively coupled plasma at thetarget; and an impedance match network coupled between the VHF generatorand the target; wherein the support is configured to provide a first VHFground return path at the frequency of the VHF generator.
 2. Theapparatus of claim 1, wherein the support is positioned such that thegap is about 60 mm and gas supply is configured such that the gaspressure is about 100 mT.
 3. The apparatus of claim 1, wherein asidewall of the chamber is configured to provide a second VHF groundreturn path.
 4. The apparatus of claim 3, wherein at least one of thefirst or second VHF ground return paths comprise a variable reactanceelement.
 5. The apparatus of claim 3, wherein one of the first or secondVHF ground return paths has impedance corresponding to an open circuitat the VHF frequency.
 6. The apparatus of claim 3, wherein one of thefirst or second VHF ground return paths has an impedance correspondingto a short circuit at a harmonic of the VHF frequency.
 7. The apparatusof claim 3, wherein the first VHF ground return path and second VHFground return path are configured such that a bias voltage on theworkpiece relative to plasma in said chamber is limited to below anupper threshold voltage corresponding to an ion bombardment thresholdvoltage of the carrier gas.
 8. The apparatus of claim 7, wherein thefirst VHF ground return path and second VHF ground return path areconfigured such that the bias voltage is held above a lower thresholdvoltage corresponding to a deposition threshold voltage of the metallicelement.
 9. The apparatus of claim 1, comprising a low frequency RFplasma bias power generator configured to be coupled to the workpiecethrough the support.
 10. The apparatus of claim 9, wherein the lowfrequency RF plasma bias power generator is configured such that a biasvoltage on the workpiece relative to plasma in said chamber is limitedto below an upper threshold voltage corresponding to an ion bombardmentthreshold voltage of the carrier gas.
 11. The apparatus of claim 10,wherein the bias voltage on the workpiece comprises a positive voltagecomponent attributable to source power applied to the target by the VHFgenerator and a negative voltage component that is a function of thelevel of RF bias power.
 12. The apparatus of claim 1, comprising a DCpower source configured to apply DC power to the target.